To initially create your chocolate sourdough starter you will need an already healthy regular starter. I now run two starters, both a sweet chocolate one and a regular one.
Add all ingredients to a jar and stir vigorously. Allow to sit on the bench for 12 hours until bubbly. You can now discard and re-feed with all the ingredients other than the original starter. It will take a couple of discards and refeeds to get this starter ready to use.